Inspection of processed (circuit covered) semiconductor wafers is commonly carried out in an apparatus wherein illumination is provided by a high-quality beam of UV laser radiation. The combination of the high beam-quality and the (short) UV wavelength allow for a tight focusing of the laser beam on the wafer for providing inspection with high-resolution.
A commonly used laser for providing the laser-radiation beam is frequency-converted solid-state laser using a neodymium-doped (Nd-doped) crystal gain-medium such as neodymium-doped yttrium aluminum garnet (Nd:YAG) or neodymium-doped yttrium orthovanadate (Nd:YVO4). Such Nd-doped gain-media have a fundamental emission wavelength at about 1064 nanometers (nm). In order to provide radiation having a wavelength of about 266 nm, this fundamental wavelength is first frequency-doubled in first optically nonlinear crystal to provide radiation having a wavelength of about 532 nm. The 532 nm-radiation is then frequency-doubled in a second optically nonlinear crystal to provide the 266 nm-radiation.
A high quality multi-element telescope for focusing ultraviolet radiation at wavelengths less than 300 nm can, practically, only include elements made from selected high grade fused silica or fluorides. This means that chromatic correction of the telescope (to the precision required of the inspection apparatus) over a useful band of wavelengths is practically not possible. Accordingly, the lens is corrected for only one wavelength, i.e., the frequency converted UV wavelength of the solid-state laser. Because of the tightness of focus required the UV laser wavelength must be within about ±0.05 nm of the wavelength for which the lens is corrected.
An advantage of the frequency converted solid-state laser in this regard is that the fundamental wavelength, and accordingly the frequency-converted wavelength, results from a very-narrow emission band of the gain-medium. The center wavelength of this band is essentially invariable over a normal range of operating conditions and environments of the laser, and does not change with operating time of the laser.
An attractive alternative to a frequency-converted solid-state laser, at least from the point of view of continuous wave (CW) output power, beam-quality, and cost, is a frequency-converted OPS-laser. Intra cavity frequency-doubled CW OPS-lasers with single-mode output of over 10 Watts (W) at green wavelengths are now commercially available. External doubling can convert this green output to UV.
An OPS-laser employs a multilayer semiconductor gain-structure including active layers separated by pump-radiation absorbing spacer layers. The gain-structure surmounts a mirror structure that provides one mirror of a laser resonator, either fully reflecting end-mirror or a fully reflecting fold-mirror. The fundamental wavelength of the gain-structure can be varied by varying the composition of the active layers. A detailed description of high-power CW OPS-lasers, both fundamental and frequency-converted, is provided in U.S. Pat. No. 6,097,742, assigned to the assignee of the present invention, and the complete disclosure of which is hereby incorporated herein by reference.
The OPS gain-structure has a relatively wide gain-bandwidth, for example about 30 nm around a center wavelength of about 1000 nm. Typically, a birefringent filter (BRF) is located in the resonator to select an oscillating wavelength from the gain-bandwidth. In the case of an intra-cavity frequency-doubled OPS-laser, this serves also to keep the oscillating wavelength within the acceptance bandwidth of the optically nonlinear crystal used for the frequency doubling.
While stability of the output wavelength of an OPS-laser is adequate for most applications, subtle variations in the OPS gain-structure, and the BRF, with environmental conditions or aging are such that control to an absolute wavelength within the above-discussed ±0.05 nm requirement for high precision wafer inspection at hard UV wavelengths cannot be guaranteed. If OPS-lasers are to be substituted for solid-state lasers in this application, a remedy for this wavelength-stability shortcoming is necessary.